Biology · Biotechnology: Principles and Processes · NEET
Yes. 'Molecular scissors' is just the simple name NCERT uses for restriction enzymes. They are proteins (enzymes) that cut the DNA strand. In a NEET question, if you see 'molecular scissors', the answer is restriction enzymes (restriction endonucleases), not DNA polymerase or DNA ligase.
Only at one fixed place. Each restriction enzyme first reads the DNA to find its own special sequence, usually 4 to 8 base pairs long (called the recognition site). It cuts only when it finds that exact sequence. So EcoRI will only cut where the DNA reads GAATTC. This is why it is called a 'restriction' enzyme - its cutting is restricted to that site.
'Endo' means inside. A restriction endonuclease cuts inside the DNA strand, in the middle, at its recognition site. An exonuclease removes nucleotides only from the ends of DNA. NEET loves this trap: restriction enzymes are endonucleases (cut inside), never exonucleases.
They are found naturally in bacteria. Bacteria make them to cut and destroy the DNA of invading viruses (bacteriophages), acting like a defence system. Scientists then isolate these enzymes from bacteria and use them as tools. Each enzyme's name comes from the bacterium it was found in (for example, EcoRI from Escherichia coli).
They do opposite jobs. A restriction enzyme (molecular scissors) CUTS DNA at a specific site. DNA ligase (molecular glue) JOINS two DNA pieces together. In recombinant DNA technology you first cut with the restriction enzyme, then paste with DNA ligase. Mixing these two up is one of the most common NEET mistakes.
Which of the following can act as molecular scissors?
Identify the wrong statement with regard to Restriction Enzymes.
The specific palindromic sequence which is recognized by EcoRI is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
There is no abbreviation - the name itself describes it. 'Restriction' means its cutting is restricted to one specific recognition sequence. 'Endo' means it cuts inside the DNA strand. 'Nuclease' means it is an enzyme that breaks nucleic acid (DNA). Together: an enzyme that cuts inside DNA at a specific site.
For NEET, know that there are two functional groups mentioned in NCERT: exonucleases (remove nucleotides from the ends) and endonucleases (cut inside at specific sites). Restriction enzymes belong to the endonuclease group. You should also remember named examples like Hind II (the first one discovered) and EcoRI.
They cut double-stranded DNA. The enzyme recognises a palindromic sequence, which exists on both strands, and cuts both strands. Depending on where it cuts, it produces either sticky ends (staggered cut) or blunt ends (straight cut).
Because they let you cut a specific gene out of any source DNA and cut the vector (like a plasmid) at the same site. Since the same enzyme is used on both, the cut ends match and can be joined by DNA ligase. Without this precise cutting, you could not build recombinant DNA.
Enzymes that cut exactly at the centre of the palindrome (like EcoRV) leave blunt (flush) ends. Enzymes that cut a little away from the centre (like EcoRI) leave short single-stranded overhangs called sticky ends. Sticky ends are easier to join, which is why they are preferred in cloning.
